Supramolecular Hydrogen-bonded Liquid Crystals Research Articles

New supramolecular hydrogen-bonded liquid crystals based on 4-alkylbenzenesulfonic acids and 4-pyridyl 4′-alkyloxybenzoates: Quantum chemical modeling and mesomorphic properties

• The new supramolecular liquid crystals with aromatic sulfonic acids were obtained. • Structural unit of systems under study is hydrogen-bonded complex. • Proton transfer occurs between sulfonic group and pyridyl fragment. • The results were confirmed by quantum chemistry, FTIR spectroscopy. • H-complexes formed by mesogen and nonmesogen have mesomorphic properties. The results of quantum-chemical modeling of the self-assembly process in the system based on 4-alkylbenzenesulfonic acids and 4-pyridyl 4′-alkyloxybenzoates are presented. It has been shown that the interaction of the pyridyl fragment of 4-pyridyl 4′-alkyloxybenzoat and the sulfonic group of 4-alkylbenzenesulfonic acid leads to the strongest H-complexes. The potential function of proton transfer from the sulfonic group to the pyridyl fragment was simulated both in the gas phase and in a solvent (ethanol). It was found that an H-complex with a hydrogen bond of the O∙∙∙H–N type is formed between the compounds under study. FTIR spectra of the 1(4-alkylbenzenesulfonic acid) : 1(4-pyridyl 4′-alkyloxybenzoate) systems confirm the results of quantum chemical calculations. The mesomorphic properties of the 1(4-methylbenzenesulfonic acid) : 1(4-pyridyl 4′-heptyloxybenzoate) supramolecular complex were studied using DSC and polarizing microscopy. It is shown that the H-complex formed by 4-methylbenzenesulfonic acid and 4-pyridyl 4′-heptyloxybenzoate has a smectic mesophase with expanded temperature range (59º) in comparison with the 4-pyridyl 4′-heptyloxybenzoate (28º).

Azobenzene-based supramolecular liquid crystals: The role of core fluorination

The impact of core fluorination on the phase behaviour of supramolecular hydrogen-bonded liquid crystals (HBLCs) is investigated in detail. Therefore, different types of HBLCs were synthesized using two benzoic acid derivatives as proton donors, namely, 4-octyloxybenzoic acid and octylbenzoic acid. The two acids were combined through intermolecular hydrogen-bonding with alkoxyazopyridine derivatives as proton acceptors. Three different types of azopyridines were used either without fluorine substitution or with one lateral fluorine substituent at different positions. The study proved the importance of using core fluorination as a significant tool to modify the liquid crystalline behaviour of HBLCs, where all azopyridines are non-mesomorphic and almost all their complexes exhibit enantiotropic mesophases. The formation of the hydrogen bond between the complementary components was confirmed using FTIR and 1H NMR spectroscopy, while the liquid crystalline self-assembly of the HBLCs was investigated in detail using polarized light microscope (PLM) and differential scanning calorimetry (DSC). Depending on the type of the terminal chain on the benzoic acid derivative and on the position of the lateral fluorine substituent different types of mesophases including nematic (N), smectic A (SmA) and smectic C (SmC) phases were observed. Finally, under UV light illumination all the prepared HBLCs show cis-trans photoisomerization resulting in tuning the liquid crystalline phases, which is of importance for industrial applications.

Supramolecular hydrogen-bonded liquid crystals based on 4-n-alkylthiobenzoic acids and 4,4′-bipyridine: Their mesomorphic behavior with comparative study including alkyl and alkoxy counterparts

We designed hydrogen-bonded liquid crystals (HBLCs) between a 4-n-alkylthiobenzoic acid homologous series and a 4,4-bipyridine at each ratio of 2/1 (mol/mol), and investigated their mesomorphic behavior. They show a wide range of mesophases compared with those formed in each single component of 4-n-alkylthiobenzoic acids. With increasing alkyl chains, they represent the following gradual changing for thermally additional phase sequences; from nematic phase to smectic (Sm) A (SmA) phase, then to SmC phase and further to SmI phase. In addition, their phase transition behavior was comprehensively discussed together with those for two familiar counterparts based on 4-n-alkylbenzoic acids and 4-n-alkoxybenzoic acids. It was found that the melting points are the highest for 4-n-alkoxy complexes and the trends of those for the rest depend on the alkyl chain length, whereas the isotropic points and the resultant mesophase ranges are clearly in the order of 4-n-alkoxy >4-n-alkyl >4-n-alkylthio complexes.

Observation of induced new smectic phase in supramolecular hydrogen bonded liquid crystals between mesogenic and non-mesogenic aliphatic compounds

ABSTRACTPresent supramolecular hydrogen bonded liquid crystal (HBLC) complex is obtained between mesogenic 4-nonyloxybenzoic acid (9OBA) and non-mesogenic citric acid (CA). A phase transition temperature and enthalpy values are calculated by differential scanning calorimetry (DSC) and phase behaviour are identified by polarizing optical microscope (POM). The liquid crystal parameters and structure are characterized by POM, DSC, FT-IR, NMR and optical tilt angle studies. The DSC and POM reveals the smectic and nematic phase transitions in the present complex. The molecular structure of HBLC complex is elucidated by Proton Nuclear Magnetic Resonance (1H NMR). The noteworthy in the present complex is the nematic and Smectic C phase is separated by a Smectic X phase which shows the finger print textures. The Smectic X phase is estimated by varying the mole ratio of the CA+9OBA from 1:1 to 1:3. Molecular tilt with respect to the director the Smectic X and Smectic C phase data are fitted to a power law equation in the optical tilt angle studies and it follows the Mean Field Theory. The 1:2 mole ratio of CA+9OBA shows monotropic Smectic C phase. Another creditable observation is that while the mole ratio of 9OBA mesogenic compound increases the nematic range also increases in the present HBLC complex.

Synthesis and mesophase behaviour of branched azobenzene-based supramolecular hydrogen-bonded liquid crystals

ABSTRACTFive types of new pyridine-based azo compounds, namely, 4-(4′-pyridylazo)-4′‘-alkoxybenzoates or 4-(4′-pyridylazo)-4′’-substituted benzoates with straight or branch terminal were prepared as hydrogen bond (H-bond) acceptors to build mesogens. After self-assembled with different carboxylic acids, the resulted H-bond complexes were confirmed for the stability of H bond by fourier transform infrared spectroscopy (FTIR) spectroscopy, and investigated for the mesophase behaviour by differential scanning calorimetry and polarised optical microscopy. The influence of terminal chains in azo accepter or acid donor and the structure of acid donors on the mesogenic behaviour were discussed in detail.

Characterization of a new smectic ordering in supramolecular hydrogen bonded liquid crystals by X-ray, optical and dielectric studies

A novel series of linear supramolecular hydrogen bonded liquid crystals (SMHBLC) has been investigated. Complimentary hydrogen bonds are formed between various alkyloxy benzoic acids. Formation of hydrogen bond is confirmed by FTIR spectroscopic studies. These homologs exhibit rich phase variance as evinced by various textures by polarizing optical microscopic (POM) studies. DSC studies revealed the transition temperatures and corresponding enthalpy values which are elucidated from the DSC thermograms. Phase diagram has been constructed through POM and DSC data. An interesting finding is the observation of a new smectic ordering labeled as smectic X. This phase has been characterized by various techniques which includes XRD. Optical tilt angle in smectic C and smectic X phases of various homologs has been measured and fitted to a power law. Dielectric relaxations have been carried out in nematic and smectic C phases of different mesogens and their activation energies are calculated. An interesting observation is the optical shuttering action of the mesogens in the nematic phase. In addition to the above phenomenon the light filtering action of these mesogens in the nematic phase has been studied.

Thermal Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals Formed by Nonyloxy and Alkyl Benzoic Acids

A novel series of supramolecular hydrogen-bonded complexes (supramolecular hydrogen-bonded liquid crystal, SMHBLC) has been isolated from equimolar ratios of p-n-nonyloxy benzoic acid (9BAO) and seven p-n-alkyl benzoic acids (nBA) whose carbon number varied from ethyl to octyl. Complementary hydrogen bonds are formed between them. They have been investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and further these data are utilized for the construction of phase diagram. Characteristic textures of the orthogonal and tilted phases have been recorded and compared. A new smectic ordering referred to as smectic X is characterized by various techniques. Odd–even effects are evinced in transition temperatures. The order of the transitions is distinguished by thermal analysis. The magnitude of experimental optical tilt angle in smectic C of all the homologs is fitted to a power law and the value of exponent is found to concur with the mean field theory predicted value.

Synthesis and Characterization of Supramolecular Hydrogen-Bonded Liquid Crystals Comprising of p-n-Alkyloxy Benzoic Acids with Suberic Acid and Pimelic Acid

Two series of supramolecular hydrogen-bonded liquid crystalline complexes have been designed and synthesized. A successful attempt has been made to form hydrogen bond between Suberic acid (SA) and Pimelic acid (PA) with p-n-alkyloxy benzoic acids (nBAO) by varying the respective alkyloxy carbon number. The first homologous series comprises of Suberic acid and p-n-alkyloxy benzoic acids (SA+nBAO), while the another series is formed between Pimelic acid and p-n-alkyloxy benzoic acids (PA+nBAO), where n represents the alkyloxy carbon number which varies from 5 to 12. These two homologous series are analyzed by polarizing optical microscope, differential scanning calorimetry, Fourier transform infrared spectroscopy (FTIR), and proton NMR studies. The formation of intermolecular hydrogen bond is evinced through FTIR and 1H-NMR. An interesting feature of PA+nBAO homologues series is the inducement of tilted smectic C phase with increasing carbon chain length. In the SA+nBAO homologues series odd–even effect is observed at isotropic to nematic transition temperatures. Phase diagrams of the above complexes are constructed and compared.

Thermal and Dielectric Investigations on Supramolecular Hydrogen Bonded Liquid Crystals

A novel series of linear supramolecular hydrogen bonded liquid crystals (SMHBLC) have been investigated. Complimentary hydrogen bonds are formed between p-n nonyloxy benzoic acid and various p-n alkyloxy benzoic acids whose carbon chain length varied from 5 to 12. The formed mesogens are characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC). Phase diagram has been constructed through POM and DSC data. An interesting finding is the observation of a new smectic ordering, labeled as smectic X, which has been characterized by various thermal and electrical techniques. Optical tilt angle in smectic C and X phases have been measured. The theoretical mean field value β deduced from the experimental tilt angle values is observed to be in good agreement with the predicted value. Dielectric studies revealed a low frequency relaxation mechanism in smectic C phase and the corresponding activation energy has been calculated from the corresponding dispersion curves. A worth mentioning observation is the optical shuttering action of the mesogens in nematic phase. Light filtering action of these mesogens in nematic phase has also been evaluated. Low pass, high pass, and band pass filtering action has been observed and discussed.

Optical shuttering action in nematic phase of SMHBLC: observation of a ribbon-like texture

In this study, complexes belonging to supramolecular hydrogen-bonded liquid crystal homologous series are synthesized and characterized. Hydrogen bond is formed between p-n-alkyloxy benzoic acids (nBAO, where n = 5–11) and chlorobenzoic acid (ClBAO), respectively. The isolated homologues are characterized by various techniques such as polarizing optical microscopic (POM) studies, differential scanning calorimetry (DSC), and Fourier transform–infrared spectroscopy. Based on the POM and DSC studies, the phase diagram has been constructed and discussed. A new smectic ordering, labeled as smectic R, has been characterized, which exhibits a ribbon-like texture. This phase is observed in the complexes pertaining to the higher homologous series. Tilt angle in this phase has been experimentally deduced and the results are fitted to the power law which concurs with the mean-field theory predicted value. Optical shuttering action in the homologue has been detected in the nematic phase and the results are also discussed.

Thermal and optical characterization of a novel series of supramolecular liquid crystals

Abstract Supramolecular hydrogen bonded liquid crystal (SMHBLC) homologous series is synthesized and characterized. Hydrogen bond is formed between p- n -alkyloxy benzoic acids (nOBA, where n =7 to 12) and Iodo substituted benzoic acid (I m BA) respectively. The isolated homologs are characterized by various techniques like Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), 1 H Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). From the POM and DSC studies phase diagram has been constructed and discussed. A new smectic ordering, labeled as smectic R, has been characterized which exhibits a ribbon like texture. This phase is observed in a complex pertaining to the undecyloxy carbon number. Tilt angle in smectic R phase and in traditional smectic C of all the complexes have been experimentally deduced and the results are fitted and so obtained β value concurrences with the Mean field theory predicted value.

Optical Shuttering and Filtering Action in Nematogens of Supra Molecular Hydrogen-Bonded Liquid Crystals

A novel series of linear supra molecular hydrogen-bonded liquid crystals (SMHBLC) have been isolated. Complimentary hydrogen bonds are formed between various alkyloxy benzoic acids. The formation of hydrogen bond is conformed by FTIR studies. This series exhibits rich phase variance as evinced by various textures through polarizing optical microscopic (POM) studies. Differential scanning calorimetry (DSC) studies revealed that the transition temperatures and corresponding enthalpy values are elucidated from the DSC thermograms. Phase diagram has been constructed through POM and DSC data. An interesting finding is the observation of a new smectic ordering labeled as smectic X. This phase has been characterized by various techniques. Optical tilt angle in smectic C phase of various homologues has been measured and fitted to theoretical values. The critical exponent β value estimated by fitting the data of tilt angle is found to agree with the Mean Field prediction. Light filtering action of these mesogens in nematic phase has been studied. It is interesting to find that the magnitude of the cut off frequencies in higher homologues is almost similar while lower homologues behave as a band pass filter. Another unique feature is the observation of a notch filtering action in one of the homologue. Yet another interesting finding is the optical shuttering action of the mesogens in nematic phase. On application of an external field at a threshold field, these mesogens inhibits light thus behaving as optical shutters. In view of the above properties, these mesogens can be exploited for commercial applications.